Français Temperature and humidity shape how fast a cannabis plant grows and how well it resists mold and stress. As a rule, give young plants warm, fairly humid air, then lower the humidity as they flower. Use the ranges below as a guide, and measure at canopy height rather than guessing.
Climate
The target at each stage
Warm and humid for young plants, drier as the flowers form.
Seeds for beginners
Ideal ranges by stage
| Stage | Temperature (lights on) | Relative humidity |
|---|---|---|
| Seedling | 22–26 °C | 65–70% |
| Early growth | 22–28 °C | 50–65% |
| Late growth | 22–28 °C | 40–60% |
| Early flowering | 20–26 °C | 40–45% |
| Late flowering | 18–24 °C | 35–40% |
| Drying | 18–21 °C | 55–60% |
These are typical ranges. Individual varieties differ, so watch your plants and adjust.
Quick targets
Keep these in view
Why the numbers matter
- Too hot (above about 30 °C): growth slows, plants stretch and aromas fade.
- Too humid in flower: damp air inside dense flowers invites mold and bud rot.
- Too dry for seedlings: young plants lose water faster than their roots can replace it.
- Large swings: a night drop of a few degrees is fine; sudden or large swings stress the plant.
Lights on and lights off
When the lights go off, the temperature drops and the humidity rises. A drop of up to about 5 °C is normal. Check the humidity at night too, especially in late flowering, because that is when damp air does the most harm.
How to measure
Place a thermometer-hygrometer at canopy height, away from direct light and fan outlets. Use one that records the minimum and maximum, so you can see what happened while you were away. Check it every day or two.
Simple fixes
| Problem | What to try |
|---|---|
| Too hot | Raise the exhaust fan speed, dim or raise the lights, and improve the air intake |
| Too cold | Move the grow space away from cold walls and floors, or add a small heater |
| Too humid | Add a dehumidifier, increase airflow, space the plants and remove lower leaves that get no light |
| Too dry | Add a humidifier, or group the plants to share humidity while they are small |
Fans that move air through the tent help with both heat and humidity. Our guide to controlling cannabis smell explains how to size an exhaust fan and carbon filter, which also sets your airflow.
Two common problems
What each one looks like
Too humid
Mold and bud rot risk
Musty smell, damp leaves and soft spots in dense flowers.
Too dry
Stressed, thirsty plants
Curling leaf edges and slow growth, especially in seedlings.
A note on VPD
Some growers use vapor pressure deficit (VPD), a single number that combines temperature and humidity. If you use it, typical targets are roughly 0.4–0.8 kPa for seedlings, 0.8–1.2 kPa for growth and 1.0–1.5 kPa for flowering. The simple ranges above are enough for most home growers.
Growing outdoors
You cannot control outdoor conditions, so choose varieties that finish before the cool, damp weather of autumn. Check the harvest month of each variety, and see our fast version seeds and autoflower seeds. For the differences between seed types, read autoflower vs photoperiod seeds.
Related guides: overwatering and underwatering, light burn vs nutrient burn and, for the drying stage, how to cure cannabis buds in jars.
Common questions
What is the best humidity for flowering? Roughly 35–45%, easing toward the lower end in the last weeks, when dense flowers are most at risk of mold.
What temperature is too hot for cannabis? Above about 30 °C, growth slows and plants stretch. Aim to stay within the ranges in the table.
Do autoflowers need different conditions? No. The same ranges work for autoflower and photoperiod plants.
Should humidity be lower at night? It tends to rise when the lights go off. Check it at night, and use a dehumidifier or more airflow if it climbs too high during flowering.
How do I measure temperature and humidity? Use a thermometer-hygrometer at canopy height, away from direct light and vents, and check the minimum and maximum readings.
